The electrical charge of the outer membrane of an individual heart muscle cell is known as the "membrane potential". During each heartbeat, the membrane potential discharges (depolarizes) and then slowly recharges (repolarizes). The waveform of this periodic depolarization and repolarization is called the "transmembrane action potential." Mechanistically, the action potential is produced by a well-organized array of ionic currents across the cell membrane. These action potentials can be recorded, or mapped, using electrodes located at the distal end of a catheter inserted into a patient's heart.
At the turn of the century, it had also been recognized that a potential similar in shape to the later-discovered transmembrane potential could be recorded if one brought into contact a first electrode with an injured spot of the heart and a second reference electrode with an intact spot. Those signals became known as "injury potentials" or "monophasic action potentials" (MAPs) because of the waveform shape.
The further development of the science of MAPs may be found in U.S. Pat. No. 4,955,382, the disclosure of which is hereby incorporated by reference. It has been recognized that local heart muscle injury is not a prerequisite for the generation of MAPs, and that application of slight pressure with the tip against the inner wall of the heart will result in the generation of monophasic action potential signals. These signals can also be recorded reliably (i.e., without distortion) from a distal catheter tip electrode by using direct current (DC) coupled amplification.
This invention also relates to the thermal destruction, or ablation of endocardial tissue. Ablation of abnormal myocardial tissue (such as arrhythmia-causing tissue) is a therapeutic procedure used with increasing frequency for treatment of cardiac arrhythmias such as, for example, ventricular tachycardia. The medical technique of ablation is discussed in G. Fontaine et al., Ablation in Cardiac Arrhythmias (New York: Futura Publishing Co., 1987), and D. Newman et al., "Catheter Ablation of Cardiac Arrhythmias", in Current Problems in Cardiology, Year Book Medical Publishers, 1989.
Catheter ablation of ventricular tachycardia was first described in 1983 as a method for destroying arrhythmia-causing tissue. Typically, a pacing catheter is introduced into the left ventricle of the heart, and manipulated until the site of earliest activation during ventricular tachycardia is found, indicating the location of the problem tissue. Electrical energy, often high voltage DC pulses are then applied between a catheter-mounted electrode and an external chest wall electrode. In this way, arrhythmia-causing cardiac tissue is destroyed.
More recently, less drastic methods than high voltage pulses have been developed, which are painful (requiring general anaesthesia), and dangerous due to arcing and explosive gas formation at the catheter tip. The use of electromagnetic energy, more particularly radiofrequency (RF) or microwave energy, is currently in popular use. RF and microwave energy, unless otherwise noted, refers to energy in the electromagnetic spectrum from about 10 kHz to 100 GHz. RF ablation, usually in the range of 300-1200 kHz, is a safer alternative to high voltage DC pulsing in which RF energy is applied to the endocardium via an catheter electrode. Tissue destruction, or ablative injury, is effected by heating generated by the RF electric field. RF ablation results in a more controllable lesion size, with no gas or shock wave formation. Ablation may also be effected with energy having microwave frequencies, from about 700 MHz to 100 GHz. Ablation may be accomplished with a small ablating electrode located at the tip of a catheter inserted into a patient's heart.
Currently, no reliable method or apparatus exists to effectively, efficiently and evenly map or ablate the atrioventricular (AV) node. The AV node is the area separating the left atrium and left ventricle of the heart, which are connected by the mitral valve. Above the mitral valve lies the left atrial wall which is smooth and contiguous with the valve leaflet.
A traditional catheter tip electrode, which is cylindrical with a rounded tip, makes poor contact with both the valve tissue and the left atrial wall tissue situated directly above it. At the atrial wall, the tip slips and is unable to maintain continuous close contact with the tissue. At the valve itself, the constant opening and closing of the valve associated with the beating heart causes the tip to slip about the valve leaflet, rendering continuous close contact virtually impossible.
Important applications of the present invention are in the areas of studying and treating myocardial ischemia and cardiac arrhythmias. In particular, the present invention permits (1) precisely locating areas of myocardial ischemia, arrhythmia and tachycardia in the AV node area by using a catheter tip whose shape adapted to stably rest at the mitral valve; and (2) treating those ischemias, arrhythmias and tachycardias using a catheter tip whose shape is adapted to stably rest at the mitral valve.